Recent results from the Cassini mission suggest that hydrogen and acetylene are depleted at the surface of Titan. Both results are still preliminary and the hydrogen loss in particular is the result of a computer calculation, and not a direct measurement. However the findings are interesting for astrobiology. Heather Smith and I, in a paper published 5 years ago (McKay and Smith, 2005) suggested that methane-based (rather than water-based) life -- ie, organisms called methanogens -- on Titan could consume hydrogen, acetylene, and ethane. The key conclusion of that paper (last line of the abstract) was "The results of the recent Huygens probe could indicate the presence of such life by anomalous depletions of acetylene and ethane as well as hydrogen at the surface."

Now there seems to be evidence for all three of these on Titan. Clark et al. (2010, in press in JGR) are reporting depletions of acetylene at the surface. And it has been long appreciated that there is not as much ethane as expected on the surface of Titan. And now Strobel (2010, in press in Icarus) predicts a strong flux of hydrogen into the surface.

This is a still a long way from "evidence of life". However, it is extremely interesting.

Benner et al. (2004) first suggested that the liquid hydrocarbons on Titan could be the basis for life, playing the role that water does for life on Earth. Those researchers pointed out that "... in many senses, hydrocarbon solvents are better than water for managing complex organic chemical reactivity. Two papers in 2005 followed up on this logic by computing the energy available for methanogenic life based on the consumption of both the organics in Titan's atmosphere along with the hydrogen in the atmosphere (McKay and Smith, 2005; Schulze-Makuch and Grinspoon, 2005). Both papers made the case that H2 on Titan would play the role that O2 plays on Earth. On Earth organisms (like humans) can react O2 with organic material to derive energy for life's functions. On Titan organisms could react H2 with organic material to derive energy. The waste product of O2 metabolism on Earth is CO2 and H2O; on Titan the waste product of H2 metabolism would be CH4. As a result of the Cassini mission, there is now abundant evidence for CH4, even in liquid form, on Titan.

Organic molecules on the surface of Titan (such as acetylene, ethane, and solid organics) would release energy if they reacted with hydrogen to form methane. Acetylene gives the most energy. However this reaction will not proceed under ordinary conditions.

This is similar to our experience on Earth. Consider a chocolate bar in a jar full of air. The organics in the chocolate would release energy if they reacted with the oxygen in the air but the reaction does not proceed under normal conditions. There are three ways to make it proceed: heat it to high temperatures (fire), expose it to a suitable metal catalyst that promotes the reaction, or eat it and use biological catalysts to cause the reaction. Biology can thrive in an environment that is rich in chemical energy but requires a catalyst for the chemical energy to be released. Such is the case on Titan.

McKay and Smith (2005) predicted that if there were life on Titan living in liquid methane then that life should be widespread on the surface because liquid methane is widespread on the surface. We have direct evidence that the surface of Titan at the landing site of the Huygens Probe near the equator was moist with methane, and radar and near-infrared imagery from Cassini have revealed extensive polar lakes on Titan, both north and south. Methane-based life would have a lot of environments in which to live.

Again, this is analogous to Earth. Life is widespread on Earth because it uses water and water is widespread on Earth.

Furthermore, because it is widespread, life on Earth, in turn, has a profound effect on the environment. For example, each spring the amount of CO2 in the atmosphere drops as plants consume it to form leaves; each autumn, the amount of CO2 in the atmosphere goes up as these leaves decompose. That is, because of the ubiquity of life, the Earth breathes: one breath in during the spring, one breath out during the autumn. Widespread life has observable effects.

Taking this logic to Titan, McKay and Smith (2005) predicted that Titanian life at the surface would consume near-surface hydrogen and that this might be detectable. The depletion of hydrogen is key because all the chemical methods suggested for life to derive energy from the environment on Titan involve consumption of hydrogen (McKay and Smith 2005; Schulze-Makuch and Grinspoon 2005). Acetylene, ethane, and solid organic material could all be consumed as well. Acetylene yields the most energy, but all give enough energy for microorganisms to live.

A few notes about liquid methane based life on Titan.

First, while such life would produce CH4 it would not be a net source of CH4 but would be merely recycling C back into CH4 - undoing the photochemistry caused by sunlight in the upper atmosphere. It does not explain the persistence of CH4 on Titan over geological time.

Second, it is impossible to predict any isotopic effect that this life might have on C. On Earth, methanogens produce CH4 from CO2+H2, or from organic material derived from CO2. The net reaction is CO2 + 4H2 => CH4 + 2H2O and thus methanogens on Earth are a net source of CH4 in a world of CO2. The enzymes that mediate these reactions create methane with a large isotopic enrichment of 12C over 13C of ~5%.

On Titan, it has been predicted that methanogens would produce CH4 by C2H2 + 3H2 => 2CH4 (eg. McKay and Smith 2005). This is obviously not a net source of CH4: it merely recycles CH4, thereby undoing the photolysis of CH4 and there is no a priori reason to expect the resulting CH4 to exhibit an isotopic shift from these reactions. The C-C bond in acetylene is strong but this by itself does not imply a strong isotopic selectivity. For example, life on Earth breaks the strong bond between the N atoms in N2 without leaving a clear isotopic effect. Thus, the istopic state of C on Titan is not relevant to the question of the presence of Titanian methanogens..

The data that suggests that there is less ethane on Titan than expected is well established (Lorenz et al. 2008). Photochemical models have predicted that Titan should have a layer of ethane sufficient to cover the entire surface to a thickness of many meters but Cassini has found no such layer. The new results of Clark et al. (2010) find a lack of acetylene on the surface despite its expected production in the atmosphere and subsequent deposition on the ground. There was also no evidence of acetylene in the gases released from the surface after the Huygens Probe landing (Niemann et al. 2005, Lorenz et al. 2006). Thus, the evidence for less ethane and less acetylene than expected seems clear and incontrovertible.

The depletion of ethane and acetylene become significant in the astrobiological sense because of this latest report of a hydrogen flux into the surface This is the key that suggests that these depletions are not just due to a lack of production but are due to some kind of chemical reaction at the surface.

The determination by Strobel (2010) that there is a flux of hydrogen into the surface of Titan is not the result of a direct observation. Rather it is the result of a computer simulation designed to fit measurements of the hydrogen concentration in the lower and upper atmosphere in a self-consistent way. It is not presently clear from Strobel's results how dependent his conclusion of a hydrogen flux into the surface is on the way the computer simulation is constructed or on how accurately it simulates the Titan chemistry.

In conclusion, there are four possibilities for the recently reported findings, listed in order of their likely reality:

1. The determination that there is a strong flux of hydrogen into the surface is mistaken. It will be interesting to see if other researchers, in trying to duplicate Strobel's results, reach the same conclusion.

2. There is a physical process that is transporting H2 from the upper atmosphere into the lower atmosphere. One possibility is adsorption onto the solid organic atmospheric haze particles which eventually fall to the ground. However this would be a flux of H2, and not a net loss of H2.

3. If the loss of hydrogen at the surface is correct, the non-biological explanation requires that there be some sort of surface catalyst, presently unknown, that can mediate the hydrogenation reaction at 95 K, the temperature of the Titan surface. That would be quite interesting and a startling find although not as startling as the presence of life.

4. The depletion of hydrogen, acetylene, and ethane, is due to a new type of liquid-methane based life form as predicted (Benner et al. 2004, McKay and Smith 2005, and Schulze-Makuch and Grinspoon 2005).

I am curious that with all the discussion of acetylene, ethane to methane, there is no mention of the necessary intermediate ethylene (ethene). If one see important compounds at each oxidation level here (methane, formaldehyde, formic acid, carbon dioxide) it would seem that ethylene would play a major role in your acetylene, ethylene, ethane, methane hydrogenation energy scheme. Where can I get more info on the relative amounts of these on Titan?
John

Colin Robinson (Jun 26, 2011 at 5:11 PM):

Chris,

A question regarding the scenario where H2 and acetylene get incorporated in a solid precipitate...

As you've mentioned, an acetylene-hydrogen mixture is a source of chemical energy. So, what happens to that energy in the long term as the precipitated materials keep building up? Does it simply sit there indefinitely? Or, could some of that energetic mixture of hydrogen, acetylene (and whatever else) eventually find its way down to the subsurface liquid water-ammonia environment, and provide an energy source for chemistry or biology there?

Colin

geopilot (Mar 31, 2011 at 2:34 PM):

years ago at the jpl huygens probe initial results conference i proposed that we look at titan as a possible post greenhouse gas life collapsed carbon saturated world rather than an early stage bio evolving planet.

people chuckled.

how does your acetelyne based life scenario fit into that possibilty as an adaption of an earlier thriving life world where that life evolved into something adapted to a post greenhouse carbon saturated world?

TitanExplorer (Oct 23, 2010 at 12:37 PM):

If a submarine went below the surface of one of Titanīs lakes and explored itīs bottom itīd indeed revolutionize our current understanding of Titan.
The submarine would simply map the entire bottom of the lake and then transmit all the information onto a sattellite which transmits the information to the unmanned mothership which in turn transmits it to Earth.
I think one possible scenario for sending an unmanned vehicle to Titan which can carry multiple sattellites would involve the use of an unmanned mothership which could be controlled by a tiny brain which has been grown in a laboratory and trained to steer aircraft and space vehicles which would otherwise be difficult for human beings to maintrain control over.

TitanExplorer (Oct 18, 2010 at 5:27 AM):

I believe that it simply is a symptom of delusions of grandeur to consider that there is some kind of planetary body somewhere which does not have biological compounds in some form. Yet I am very well aware that this article is not saying that there is no life on Titan. What is being asked , is if there is. I have my own belief , and it may seem extreme to most of you so I keep it to myself and do not ask others to aggree. I believe that there is not only life on Titan , but people. I also believe the same about Mars , the Moon and so on. Since people can visit each other on this planet , it is only reasonable and perfectly logical to assume that advanced civilizations on Earth visited other planets in this solar system and perhaps behound in mankindīs remote past , and that the same may be happening on Earth today. I donīt consider life to be original to Earth , because I believe that the Earth does not have the ability to generate multicellular organisms solely on itīs own terms - they all arrived here one way or another. With single - cellular organisms there may be another matter. There are , in my honest to goodness opinion , matter of factly , no such planetary bodies as those which may not have life in some form on their surfaces.
It is my belief on the extent of physical evidence that life exists abundantly not only in this solar system but beyound , that the evidence itself is cyclopean in scope and magnitude. In other words , itīs infinite. So has the Law determined , that no planetary body is to be permitted to be born , which can not produce life , consciousness or some kind of biological compounds , etc. in some form. This is simply because truth is truth.
That is the Law.

Colin Robinson (Oct 11, 2010 at 6:32 PM):

Chris

Is this a possibility to consider in planning future missions to Titan - a naked gene able to catalyse, or a naked catalyst able to propagate itself - that might made of something other than RNA?

Colin

cmckay (Oct 4, 2010 at 4:58 PM):

Colin, it is certainly possible to see life as catalysts way of propagating themselves. If we view it this way then a catalyst needs a system that can incorporate energy and create new catalysts. Catalysts are produced by the rhibosomes in cell using energy derived from ATP and based on construction instructions encoded in the DNA and translated into RNA. In other words a cell. Catalysts can make chemical reactions go but they cannot make replicate themselves. At least that is the way it is with life today. The catalysts are made of protiens and the molecule the encodes for their formation is DNA. However it may not have been this way always. These is speculation that before there was DNA/protien life there was what is known as the RNA world. RNA can act both as genetic material and as a catalyts. So in principle there could have been a simple type of life that was effectively a naked gene that could act as a catalyst, or viewed another way a naked catalyst and could also encode its own replication. I think we would still call this life. -Chris

carolyn (CICLOPS) (Oct 4, 2010 at 11:35 AM):

Colin,

Very good point, if you ask me. Let's see what Chris has to say about it.

Colin Robinson (Oct 3, 2010 at 6:07 PM):

Chris,

Your article mentions that when we eat chocolate, we use catalysts to get energy out of it,

Which (by the Occam's Razor principle) seems to support the conclusion that Titan more likely just has catalysts, rather than catalysts plus chocolate-eaters making use of them.

But maybe there is another way of thinking about the relation between organisms and catalysts. Rather than living cells using catalysts, what if we think in terms of the catalysts using the cell as a space to work in?

Natural carbon-based catalysts here on Earth seem to need these little workshops to do their catalysing.

To assume that Titan's catalysts need no such spaces is to assume that Titan's ones are smarter, at least in the sense that we speak of smart machines.

TheAnt (Aug 8, 2010 at 4:31 AM):

Thank you Mr Chris McKay for the explanation that carbon isotope measurements might not be able to provide one answer if it is chemical catalysts or ongoing prebiotic chemistry that would explain the appearant lack of acetylene and ethane.

I am afraid I have to agree with you illexsquid, too bad about the reporting from the Telegraph and others, such writers cause more harm than good.

Well the fact still remains that Titan is the most interesting world in the solar system, I am personally convinced that such a grand chemistry lab as Titan could give us important clues both for the origin of life on Earth and possibly also for a different genesis elsewhere (Methane, ethane or acetylene around a red star for example).
So yes, we do need to get some good thinking done on how to get a well equipped orbiter / lander mission for Titan. (I advocate multiple landers to spread the risks for such a long mission, than over a one shot blimp/ballon. Too many uncertainies, not to mention tholins raining down and sticking to the ballon having it crash in a short time.)

cmckay (Jul 11, 2010 at 10:13 PM):

Colin, we are not sure about Titan's past. Prior to the Cassini/Huygens mission we assumed that Titan's atmosphere was like we see today for the age of the Solar System (3.8 Gyr). The fact that photochemistry is destroying methane implied a very big initial inventory and a concomitant pile up of ethane the main photochemical product. Hence the notion of a global ocean on Titan. Now that we see there is not ocean we're a bit puzzled as to where the methane is coming from and where the ethane is going. An easy answer for the methane is some sort of volcanism - cryovolcanism. But no good evidence for this has not been seen. I have been thinking lately that maybe Titan was a snowball world (like Triton) until recently. What we are seeing now is a recent runaway greenhouse of sorts. In this case life, or chemistry, would not have had very long to get going... Too many unknows. -Chris

Colin Robinson (Jul 6, 2010 at 5:49 AM):

Questions for Chris... I know there has been some discussion about how Titan might change in the future. But what about Titan in the past? If its atmosphere used to be substantially richer in hydrogen atoms (more H2, and perhaps NH3 instead of N2) what would that mean for how easily acetylene and ethane could be hydrogenated into methane? Could less effective catalysts have done the job, back then, than would be required now? And would molecules with catalytic properties have been more, or less, likely to form in such conditions? Has anyone experimented with this? Is it possible that an evolutionary scenario -- more sophisticated catalysts gradually appearing as an adaption to decreasing availability of hydrogen -- will turn out to be the most economical explanation? Even though that scenario of course implies existence of systems capable of evolving, and thus meeting one definition of life?

larryy (Jul 1, 2010 at 4:48 PM):

sobrient60 (Jun 9, 2010), robin (Jun 9, 2010 at 9:09 AM): Possibly most here are aware of it, but I thought it worth mentioning the (Farmer et al. 1986) results examining the likelihood of formation of autocatalysis in random reaction sets. Quoting from the abstract, "When the initial set exceeds a critical diversity, autocatalytic reactions generate large molecular species in abundance. Our results suggest that the critical diversity is not very large." Basically, cycles in the graph of chemical reactions become very likely, very quickly with graph node and edge counts. And then it's off to the evolutionary races. It's an encouraging result for finding at least simple life in even the starkest of environments. As long as there are any chemical reactions proceeding, and clearly there are on Titan, then there's at least some hope. Kauffman published a good deal more on this and related topics.

NeKto, the idea that ice might act as the catalyst is interesting and something to check out in the laboratory. We are planning to do some experiments to see if we can get C2H2 and H2 to react in Titan simulations so ice is something we will inevitably have in the mix. We'll try a control with just ice (no Titan tholin) and see what we get. -Chris

NeKto (Jun 29, 2010 at 3:34 PM):

i hate to say this, but i have come up with a hypothesis that explains the atmospheric chemistry near Titan's surface. No biology is involved.
i was doing some thought experiments along the lines of metal catalysts when i remembered that ice has different forms in cryogenic temperatures than it does at our "Earth normal" range. what i came up with is the possibility that water ice might be a catalyst for the reactions that "hydrogenate" the unsaturated hydrocarbons produced in the upper atmosphere.
if the surface of the ice has a crystalline structure that places oxygen atoms on the surface in proper alignment, there might be sufficient attraction to form hydrogen bonds, thus forming weak bonds with the hydrogen gas in the atmosphere. Carbon is reactive with both oxygen and hydrogen. if there is sufficient attraction, the unsaturated hydrocarbons might "stick" to the molecular surface already "coated" with hydrogen. if this catalyzed a reaction, is should put heat into the ice "rocks" that is not in the tar sands.
At night this should make the ice warmer than the tar.
is there a way to test that?
i do not know enough cryo-chemistry to know if this hypothesis is feasible, but i know that, if it is, several possible impurities in the ice might make it more likely. Alloying agents if you will. So, if you can shoot this one down, please do. i would rather find some simple biology.

girlspace (Jun 25, 2010 at 6:32 AM):

thank you cmckay, so that mean that maybe in nearly future peoples can live in titan i'm right? all goods to everyone

NeKto (Jun 16, 2010 at 1:19 PM):

Thank You Chris McKay.
as i said earlier, fascinating discussion!
Energy availability, i had not considered that. It is another important factor determining the possible rate of evolution and the complexity of life. It does place a rather low ceiling on any hypothetical biological activity.
another factor i suspect might limit the possibility of life on Titan is, what are available for what life here utilizes as micro nutrients? Life here has the vast majority of the periodic table available. as an example we need everything from iodine to zinc to stay healthy. And liquid water is far closer to a universal solvent than liquid methane.
But any process i can hypothesize, living or not, requires chemistry more complex than what can be expected from cryogenic water ice, hydrocarbons, nitrogen and ammonia. What else is out there? Is there any information on what "impurities" are in those ice boulders or the "tar sands" we see in the Huygens images?
How ever the "hydrogenating" of carbon compounds near the surface is being accomplished, the process must be using something to catalyze the chemistry. There has to be something very interesting going on.

Craig (Jun 16, 2010 at 9:59 AM):

That the H2 is more an analog to O2 than to CO2, ie that it's a breathing/energizing gas for animal life, is indeed what I was saying.

Visually though, from space one sees the Amazon rainforest canopy without seeing any of the animals in it. Plant life will be far more evident than animal life.

Colin Robinson (Jun 14, 2010 at 8:49 PM):

Chris. Thanks for your reponse! Yes, I enjoy the thought of life on Titan also. Are the findings about hydrogen, acetylene and ethane the chemical footprints of life? At any rate, it would seem more likely that the carbon cycle depends on an organic catalyst rather than a metal one, given that Titan's surface (unlike that of Mars) is rich in organics rather than in compounds of metals. Have you considered that the answer might lie between the non-biological explanation and the biological one? Is it conceivable, for instance, that there are systems of catalytic molecules which are carbon-based without being enzymes, and those systems can maintain themselves and observably affect their environment without necessarily having all the bells and whistles of a living cell? -Colin

cmckay (Jun 14, 2010 at 7:11 PM):

Craig, the analogy with Earth and oxygen is probably better done with heterotrophs (animals) rather than phototrophs (plants). The hypothetical life on Titan is eating organic material produced by sunlight but not by "plants" but by sunlight driven chemical reactions in the upper atmosphere that produce organics. For them its manna from heaven. Chris

JimRinX, the idea of creating a simulation chamber and seeing if life can be coaxed to come forth is an appealing one. The problem is that we have been trying this on Earth like life since the Miller-Urey experiment more than 50 years ago and so far at least, no joy. -Chris

NeKto, my intuition is the same as yours as to the tempo of life on Titan - the slow lane. However I would bet against any multicellular life. The energy yield is too low. Although we make the comparison with O2 and organics, the energy yield from H2 and organics is much lower. -Chris

Colin Robinson, indeed! one of the reasons Titan is so interesting is that it has cycles the remind us of Earth: Wind, clouds, storms, and rain, seasons, dunes, lakes, etc. The carbon cycle of the atmosphere and surface are also evidence of the activity of this place. These cycles and Earth-like processes are indeed fascination. But I do enjoy the thought that the most fascinating cycle of all - life - may be operating there as well. -Chris

Colin Robinson (Jun 14, 2010 at 2:57 AM):

To Chris McKay: I can understand the note of caution in your article... But it does look like you and your colleagues have found something that is indeed (as you put it) "extremely interesting"... Even if the explanation isn't a population of organisms... Levels of ethane, acetylene, as well as hydrogen all seem to imply that Titan's carbon chemistry is not a one-way street (compounds forming in the atmosphere, sinking to the ground and accumulating there), but a true carbon cycle. This in a world that also has a liquid cycle of evaporation and precipitation... Isn't a carbon cycle important in itself? Which other place in the solar system has a liquid cycle plus a carbon cycle?

NeKto (Jun 12, 2010 at 6:12 PM):

This is a fascinating discussion!
For the sake of argument let us presume a case where there is some form of life on Titan. No mater what the specifics of the biochemistry involved, there is a very high probability the reactions will be exponentially slower than biochemical reactions at temperatures found on Earth. Therefore, i hypothesize any life on Titan will have had exponentially fewer generations to evolve than it's terrestrial counterparts.
If there is life on Titan, i strongly suspect it will be far more primitive than life on earth. The most likely form my imagination conjures up is something analogous to a cryogenic slime mold, but i wouldn't rule out something as complex as a trilobite.
it will be fascinating to see where the data leads in this investigation.
Ya gatta love those great little robots out there! they have sent us data that has led to some of the best and most interesting science in centuries.

JimRinX (Jun 12, 2010 at 9:40 AM):

What we need to do here is experiment - thinking like the guy who detected the xtremophiles in the south african gold ore vein, by constructing a box that would maintain the (deadly - at least for US) conditions within said vein, after he'd collected them and taken them back to the lab.
If there is some kind of life on Titan, than things are looking up for panspermia; and it may be that more than just Earth-type life (archea; with, say, polychaeate methane-hydrite worms being a 'first animal' evolutionary spin off) is transported about by comets and asteroids.
Thus, we should create a chamber that recreates Titan - or even Europa (all those things growing in Antarctic Ice - not to mention Chlathates!), and then 'innoculate' it with material from, say, a 'pristene' Antarctic Chondrite - or even some 'Stardust' Dust; then see what happens.
It's also interesting to note that Metabolism, in Bacteria, has been proven to occur - 80C.; that, at those temps, a few water molecules, in small pockets, remain unbound to the crystalline latice (they stay 'liquid') - possibly accounting for the former; and that an animal life form was recently dredged up from one of the Mediteraneans Deep ANOXIC Zones.
It had eggs...it was clearly alive....it did NOT, somehow, need Oxygen to have more than one cell......etc..

Craig (Jun 11, 2010 at 9:56 PM):

If the H2 is analogous to CO2 rather than O2, then vegetation taking it up could explain its absorption at the surface pretty nicely.

On the other hand, it would seem H2, like O2, will easily react to release energy, whereas normal (unstretched) CO2 is usually an 'end product' of reactions, hard to react further to get energy out of. On Titan I expect that 'end product' would be methane(?):

C + O2 -> CO2 (solid on Titan)
C + 2H2 -> CH4 (liquid)

In the absence of another energy gas on Titan, I'm expecting H2 would be analogous to O2, though it may be there's no direct parallel - H2 appears to be being absorbed rather than produced (by plants) at the surface judging by the findings.

robin (Jun 10, 2010 at 4:21 PM):

Just for context, the abundance of CO2 in the Earth's atmosphere -an essential fuel for photosynthesis- is only 0.04%, so 0.1% is not so bad. Also, large creatures did not emerge on Earth till Oxygen levels had risen substantially and there has been speculation that those are related, that big creatures need rich concentrations of energy whereas microbial life can eke out a living with less. Partly for this reason, though I'm hoping for microbes (being a microbiologist myself), I would bet against Titanian Kraken swimming the depths of Kraken Mare, the Methane Sea near Titan's North Pole.

Craig (Jun 10, 2010 at 1:17 PM):

Thanks Chris,

That explains all!

0.1% is a lot less than 20+% O2 on Earth, but it seems the best bet on Titan for a breathing/energizing gas as far as I've seen.

Craig

cmckay (Jun 10, 2010 at 12:14 PM):

Craig, the GCMS team had difficulty extracting the H2 from the data because H2 was used a the carrier gas in the GC (gas chormatograph). The expected value from remote sensing for H2 in the lower atmosphere of Titan is 0.1%. Recently the PI of the GCMS, Hasso Niemann, presented some preliminary results of the H2 and the concentration appeared to agree with this value. -Chris

Craig (Jun 10, 2010 at 9:40 AM):

In the 2005 article "The abundances of constituents of Titan's atmosphere from the GCMS instrument on the Huygens probe", I was puzzled to see no mention of H2 actually being present in the atmosphere, given previous estimates of 0.5% and feeling it was likely to be biologically important, perhaps a breathing gas. It was indirectly implied, but I was left with the impression it must be pretty insignificant.

Just how much H2 was actually found, say, near the surface and at a given altitude or two? PPB? PPM? whole %s?

Craig Carmichael ("Living Titan" website)

robin (Jun 9, 2010 at 10:00 AM):

Just as an aside, I came to this list through an odd route. A physicist-songwriter friend of mine goaded me into promising to write a song back in February. As an old chem major, I'd been interested in Titan for a long time, and had been following the news about the proposed Titan Mare Explorer mission to "sail" one of the moon's methane seas. Lacking any other ideas, I decided to write a sea-chanty set on Titan. After I finished it, I thought maybe some real Titan-ologists would get a kick out of it, so I sent it to Carolyn Porco, who wrote back to say she enjoyed it and put me on the ciclops mailing list (which I hadn't known about). In the process of finding her contact information, I looked up her published papers, and from the author list realized that when I was an undergrad studying chemistry at Caltech, she was a grad student modeling planetary rings and her thesis advisor was the RA for my student house! (I remember him talking about the work, but did not remember the name of the grad student doing it). Small world. (Just in case anyone is interested, the song is "The Shoals of Kraken Mare" and is up on YouTube: http://www.youtube.com/watch?v=AaWg7Wm-IwM ) But back to my first comment: many thanks to Drs. McKay and Porco for doing this and please keep it up!

robin (Jun 9, 2010 at 9:41 AM):

One of the (many) things that makes Titan so interesting for pre-biotic chemistry is that it is so heterogeneous in terms of chemical environments, where lots of interesting molecules can arise. There will be non-thermal photochemistry at the top of the atmosphere (to a first approximation temperature-independent), high-temperature aqueous-phase chemistry deep underground (assuming theories about subsurface liquid water are correct), and mineral-catalyzed reactions at the base of methane lakes and pools undergoing repeated cycles of flooding and drying (and that's just a start!). As long as there is even a little mixing between these environments (and a lot of time) there will be a lot of opportunity for very complex molecules to arise. Since we only have an N of 1 (life on Earth) we just don't really know how hard it is to get from there to the autocatalytic "hypercycles" of molecules where selection can start to work and climb the complexity ladder up toward life, but Titan will be a good test case. One further interesting twist is that if there are water/ammonia cryovolcanoes (as has been hypothesized), the environment in an around them may be well within the temperature, pressure, and chemical composition where some Earth extremophiles could live -sort of a Titanian twist on the deep vents of Earth's oceans.

cmckay (Jun 9, 2010 at 9:09 AM):

sobrien60, this is a very good point. The reaction rate issue does come into play when considering prebiotic evolution that leads to the first life form. It would be good if we really understood how life did arise even for the case on Earth.

sobrien60 (Jun 9, 2010 at 8:00 AM):

The problem is there is no evolutionary path to get to your enzyme catalyzed ecosystem. Enzymes don't appear out of nowhere, they evolve by chemical reactions. If you step back to earlier and earlier epochs you reach a point where the evolutionary path is dominated by non-catalyzed chemical reactions with typical pre-exponential factors and at 90K those reactions are turned off.

cmckay (Jun 8, 2010 at 7:35 PM):

Jon, more on benzene in reply to your questions. There are three points that are relevant. 1) Benzene was detected in the upper atmosphere at ~1000 km elevation as Cassini flew through Titan's atmosphere. The concentration was about 2 parts per million as determined by Waite et al. 2007. 2) Benzene was detected in the vapor produced when the inlet of the probe was pressed against the ground after landing. I am not aware of any published quantitative values but it could also be at the ppm level. The third point is a calculation we did that showed if you take the solid organic material produced in Titan's atmosphere and isolate it and bring it to full thermochemical equilibrium what you get is benzene and N2 gas. However it may be a bit missleading to think of benzene as a chemical dead end. As robin pointed out in the first message, aromatic compounds at the top of the atmosphere are interesting and these almost certainly come from benzene.

cmckay (Jun 8, 2010 at 7:25 PM):

Jon, robin and sobrien60,

robin's explanation is exactly how I think of it. If there chemical potential energy that is not released due to slow rates or barriers then life can make use of this by catalyzing the reaction.

-Chris McKay

jonk (Jun 8, 2010 at 5:51 PM):

to robin: I'm not a chemist, but your point about catalyzed reactions makes immediate, clear sense in dealing with sobrien60's objection. Thanks.

to mckay: I'm not entirely sure I understand your response about benzene. What I gather from robin's question is the idea that benzene may be the eventual trash heap of carbon, making it permanently inaccessible afterwards. While you seem to agree with this assessment about benzene, your point about its creation high in the atmosphere and presence on the surface doesn't discuss __quantities__ in both spheres relative to each other and relative to the larger situation as well and I think that is very important. Could you expand a little? Thanks.

Jon

robin (Jun 8, 2010 at 2:50 PM):

Putting on my biochemist's hat, I think sobrien60 is missing the point McKay is making. _Un-catalyzed_ reaction rates drop with temperature, but that says nothing about what enzyme-catalyzed reaction rates might be. This is limited only by how far they might be able to lower the free energy of the transition states. The lower than predicted levels of acetylene and _maybe_ hydrogen _might_ imply that something is catalyzing an energy-producing reaction between them. Also, one shouldn't discount the potential for kinds of chemistry that is _too_ reactive for Earth biology but might work in colder climes: boron chemistry, fluorine chemistry, free-radicals, etc. Finally, there is photochemistry at the top of the atmosphere and thermal chemistry deep under the surface that will gradually mix into the surface chemistry. The point of the hypothesis (as far as I understand it) was exactly that anomalous patterns that wouldn't make sense in terms of ordinary chemical thermodynamics might actually be a clue indicating the presence of biological chemistry in the way that Sagan proposed that simultaneous presence of Methane and Oxygen in an atmosphere would be a telltale sign of life.

sobrien60 (Jun 8, 2010 at 2:14 PM):

Chemical reaction rates drop by roughly 2x for every 10 degrees of cooling. Dropping from room temp (near 300 K) to the surface temp of Titan (90 K) would drop chemical reaction rates by roughly 2^21 or over a factor of 2 million. Arrhenius rates would drop even more dramatically. Chemical reactions used by life forms would be essentially turned off by this cold environment. Life could survive in hibernation mode, but it certainly could not thrive and reproduce.

carolyn (CICLOPS) (Jun 8, 2010 at 2:05 PM):

dholmes: Well, to be fair, the imaging system says nothing about the kind of chemistry that is at the heart of these inferences. This latest output was the work of others, and not mine. But I'm certainly an enthusiastic cheerleader!

dholmes (Jun 8, 2010 at 12:27 PM):

Tp further comment on possible methane life forms maybe they are like earth's prolific worms like the ones that live near hydrothermal vents in the ocean at temperatures that can exceed 100 degrees Celsius (212 degrees Fahrenheit), or worms living in ice on Alaskan glaciers at zero degrees Celsius (32 degrees Fahrenheit). In other words life seems to adapt, a fact that may be to the extreme on Titan. It is nice however that the strongest possibility of life yet so far found has come to us through Carolyn's work with Cassini.

dholmes (Jun 8, 2010 at 5:40 AM):

In answer to rdstancy, to my knowledge no, but an excellent idea all the same. But what life are we talking about, microbial or more complex?

cmckay (Jun 7, 2010 at 10:38 PM):

to: rdstacy. Any methane life studied on Earth?
No All life forms studied on Earth require liquid water to grow or reproduce. There are organisms that eat methane and others that produce methane but Earth is too warm for there to be any naturally occurring liquid methane.

to: illexsqui. The variations used by Strobel. Strobel did a good job of considering a variety of possibilities especially with respect to chemistry. He even showed under what conditions the flux he was postulating would not be present - that would be the case if the concentration of hydrogen at the surface was twice what we currently think, about 0.2% instead of 0.1%. I emailed a colleague who is an expert on this measurement to ask him if its possible.
next step: confirm the hydrogen conclusion and Cassini may help here, then go to Titan and get more data with something that flys, swims, or lands.

to: robin. benzene? Yes indeed Benzene is a mystery player in Titan atmosphere and on its surface. It is formed high in the atmosphere - to our surprise, and it was found in the surface - again to our surprise. It is also the thermodynamically stable end form for the solid organic produced in the atmosphere.

to: billclawson. DNA? Unlikely that non-water-based life would use DNA as its genetic material. DNA would probably unwind in a non-polar solvent like liquid methane.

billclawson (Jun 7, 2010 at 4:38 PM):

Fascinating idea. So if the measurements aren't a mistake and there is life on Titan, it's certainly well outside of the "life as we know it" norm. What sort of "DNA" might such a life-form carry?

robin (Jun 7, 2010 at 2:30 PM):

Thanks for the article. Great stuff. As a scientist, I find most news articles frustratingly vague (or wrong!) about what the data really are and what they mean. Especially appreciate the citations. As an old organic chem major, I've been interested in Titan chemistry for a long time. Quick question: Aside from Methane itself, the one organic molecule I've seen cited as present in much _higher_ than expected abundances on Titan is benzene. Might this be another biomarker, since methane organisms could not get energy by reducing it with H2 (since it is so stable from resonance effects)? That is, in a methane ecology would there be an excess of carbon atoms that eventually end up in a benzene dead-end? (As an aside, I think aromatic compounds will be very interesting on Titan with antiaromatic-transition-state-preferring photochemistry at the top of the atmosphere, aromatic-transition-state-preferring thermal chemistry deep underground, and slow cycling of carbon atoms between them.)

illexsquid (Jun 7, 2010 at 1:23 PM):

The clarity and measured optimism of this article are a voice of reason that needs to be heard. It is unfortunate that an interesting new finding gets so obscured by the time it passes through the filter of the mainstream media. Honestly, do "science reporters" these days have to know even basic high school science?

Not having seen the Strobel paper, the thing I was left wondering is this: how much variability in the paramaters of their model did they allow, and how did this affect the resultant expectation of hydrogen flux? Presumably, his team ran more than one simulation, and was able to come up with some determination of how sensitive the model is to varying conditions.

Also, presuming this result holds, what is the "next step"? Does Cassini have any more instruments that can bolster or refute this claim? Or do we have to wait for the promised Titan balloon mission?

rdstacy (Jun 7, 2010 at 12:29 PM):

Has a methane-based biological system ever been studied in the laboratory on earth at Titan's temperature ?